THE NEXT time you watch a major sporting event, the chances are that more than half the participants will have used the controversial food supplement creatine. Nearly 57 per cent of the country's leading sportsmen and women have used the product, according to The Independent's survey.

Creatine is a substance found naturally in the body and in foods such as meat. It helps build muscle and aid recovery. In recent years, an increasing number of sports participants have been using commercially produced creatine, including the England World Cup football team.

Gary McAllister, the Coventry City and Scotland footballer, used creatine following last December's knee ligament damage that kept him from playing in the World Cup. He was playing again by September and believes creatine helped him recover more quickly. "It certainly allowed me to train longer, though there is a weight gain from it. You have to be in really strict training," he said.

Swimmer Mark Foster, a gold medal winner at this year's Commonwealth Games, said after using creatine: "Mentally I don't feel any different, but I do feel I've got more energy."

Doctors have found that heavy use of the substance can seriously aggravate hidden kidney problems, and also fear that extended use could lead to toxins being absorbed into cells because it encourages the body to absorb water.

Published reports have noted a number of adverse effects, including involuntarily clenched teeth, diarrhoea and "the sound of blood rushing in the ears". Furthermore, no long-term clinical test has been carried out to assess the use of creatine for more than a year - although Dr Annette Hudson, who has studied its effects, pointed out: "Anyone who is on a high-protein diet is on a high-creatine diet at the same time; that's the long-term natural study."

Creatine occurs naturally, in tiny amounts, in lean meat such as beef and pork, fish such as tuna, salmon, herring and cod, and loganberries. A 70kg (11-stone) person stores about 120g of creatine, almost all in the skeletal (rather than heart or intestinal) muscles. It can also be made by the liver using amino acids from proteins.

Creatine acts as a "reservoir" for extra energy in the muscle. To contract, muscles use their limited stores of a chemical called ATP. When that is exhausted, creatine is used to make extra ATP to extend intense bursts of energy. Furthermore, because it is stable within the body - unlike ATP, which must be used quickly once made - it can help recovery after intense exercise.

Dr Nick Pritchard and Dr Philip Kalra reported in The Lancet medical journal earlier this year the case of a man who was successfully treated for minor kidney problems eight years ago, but in June last year had high levels of creatine - which can indicate kidney stress. The man told the doctors he had been taking creatine supplements for eight weeks as part of his pre-season soccer training. Dr Pritchard commented that there was "strong circumstantial evidence" that the substance caused the man's renewed kidney difficulties.

Dr Hudson, who is a technical adviser to the Weider bodybuilding company, said: "You can overdose on anything, even headache pills. If you have a family history of kidney problems, discuss it with your GP: certainly if you do then using creatine will raise your blood pressure."

A parallel concern is that the muscles swell with water when they absorb creatine. Dr Ross Bailey, who heads the sports training department at Texas Christian University in Fort Worth in the United States, noted that: "Use of creatine tends to increase the permeability of the body's cells to certain toxins." He recommended that athletes should stop using it for four weeks after at most eight-weeks use.

Conor O' Shea, an Irish rugby union international, said: "We've been advised to go on it for a month or two and then off it for six weeks to make sure we're not at any risk from side-effects." He added: "It's certainly in the back of my mind that it does no good at all, but I'll give it the benefit of the doubt."

One who will not give creatine the benefit of any doubt is Andy Robinson, the coach at Bath rugby union club."I have read studies and taken advice from acknowledged experts and it seems obvious to me that more research needs to be done before athletes can safely take creatine supplements," he said.